htonl: New module. * modules/htonl: New file. * lib/arpa_inet.c: New file. * m4/htonl.m4: New file. * modules/arpa_inet (Files): Add lib/arpa_inet.c. (Depends-on): Add extern-inline. * lib/arpa_inet.in.h (htons, htonl, ntohs, ntohl): New declarations. * m4/arpa_inet_h.m4 (gl_ARPA_INET_H_DEFAULTS): Define REPLACE_HTONL. * doc/posix-functions/htons.texi (htons): Mention the module. * doc/posix-functions/htonl.texi (htonl): Likewise. * doc/posix-functions/ntohs.texi (ntohs): Likewise. * doc/posix-functions/ntohl.texi (ntohl): Likewise.